1. Field of the Invention
The present invention relates to a protection circuit for protecting confidential information stored in a semiconductor device from fraudulent analysis and to a semiconductor device including such a protection circuit.
2. Background Art
In recent years, circuitry information and internal information of semiconductor devices require a considerably high level of confidentiality and secrecy. Especially in the field of IC cards, the safety of semiconductor devices is an essential feature and, therefore, it is necessary to protect important information from fraudulent analysis and prevent tampering and copying of internal information. In view of such, an increasing number of methods for achieving such heavy protection have been proposed. Hereinafter, a typical one of such conventional techniques is described.
FIG. 14 shows the structure of a conventional protection circuit. In FIG. 14, reference numeral 140 denotes a shielded line, reference numeral 141 denotes a signal generator, reference numeral 142 denotes a detector, reference numeral 143 denotes a reference line, and reference numeral SO denotes an alarm signal. In this protection circuit, the shielded line 140 is provided over an integrated circuit to be protected. The signal generator 141 gives a signal to the shielded line 140 and the reference line 143. The signal supplied from the signal generator 141 reaches the detector 142 after passing through the shielded line 140 and the reference line 143. The detector 142 compares the signal supplied through the shielded line 140 and the signal supplied through the reference line 143. If a difference is found, the detector 142 outputs the alarm signal SO. In response to the alarm signal SO, the integrated circuit to be protected shifts to the safety mode such that fraudulent analysis and tampering are practically impossible (see, for example, Japanese Unexamined PCT National-Phase Patent Publication No. 2002-529928 (FIG.1)).